1. Field of the Invention
The present invention is directed to a noise-reducing arrangement for a magnetic resonance tomography apparatus.
2. Description of the Prior Art
During operation of a nuclear magnetic resonance tomography apparatus, a magnet assembly generates a static basic field on the order of magnitude of approximately 1 Tesla. For imaging, it is necessary to superimpose magnetic field gradients on this basic field. This occurs with a gradient coil assembly that is arranged in the basic field. The gradient coil assembly usually includes a number of coils in order to generate three magnetic field gradients residing perpendicularly relative to one another.
Each gradient coil is typically permeated by a pulse-shaped current on the order of magnitude of up to 300 A with rise times of less than 1 ms. Lorentz forces that cyclically change according to the flow of current through the gradient coils thereby act on the conductors of the gradient coils in the basic magnetic field. These forces cause the gradient coil assembly to oscillate, causing a knocking noise unpleasant for the patient to arise, which can even reach the pain threshold in extreme cases.
U.S. Pat. No. 5,345,177 discloses a nuclear magnetic resonance tomograph of this type wherein a tubular gradient coil assembly has both ends connected to the inside wall of a cryostat by damping means. The damping means has one or two retainer pins that reside in essentially punctiform contact with the gradient coil assembly or the cryostat.
U.S. Pat. No. 5,256,969 discloses a gradient coil assembly having a specially stiffened and oscillation-damping coil body. Details as to the connection of the gradient coil assembly to a magnet assembly are not disclosed.
U.S. Pat. No. 5,331,281 discloses a gradient coil assembly whose natural frequency deviates from the frequency of a force acting on the assembly in order to reduce the creation of noise.
German OS 195 31 216 discloses a nuclear magnetic resonance tomography apparatus wherein the gradient coil assembly is elastically suspended in the region of a oscillation node to be expected during operation. The elastic suspension is relatively soft because it is intended to damp the transmission of oscillations of the coil assembly onto the magnet assembly. The oscillatory properties of the gradient coil assembly therefore are not modified significantly by the elastic suspension. A considerable noise level thus continues to occur during operation of this known tomography apparatus.
German PS 41 41 514 shows a gradient coil assembly with two shells that are concentric and connected to one another with a press fit. The assembly is especially rigid in mechanical terms, and as a result the sound pressure level in the examination space is reduced, however, this assembly is complicated to manufacture.